Transcript Absorption & emission line spectra

The Sun crossed the celestial
equator heading south at 11:09
EDT last night. Known as an
Autumnal equinox, this
astronomical event marks the
first day of autumn in the
northern hemisphere and spring
in the south.
This image of the Sun in
extreme ultraviolet light was
recorded yesterday with the
Solar Dynamics Observatory.
The false-color image shows
emission from highly ionized
iron atoms. Loops and arcs
trace the glowing plasma
suspended in magnetic fields
above solar active regions.
Homework #3 is due Tuesday, Sept. 28, 5:00 pm
– extra credit question due at class time that day
Exam #1, Thursday, Sept. 30
Review session: Tuesday (9/28), 7:00 pm.
Light as a Particle
 Light can also be treated as photons – packets of energy.
 The energy carried by each photon depends on its frequency
(color)
 Energy:
E = hf = hc/  [“h” is called Planck’s Constant]
Shorter wavelength light carries more energy per photon.
The Electromagnetic Spectrum
lower
energy
higher
energy
Light as Information Bearer
Spectrum: light separated into its different wavelengths.
Spectroscopy: The quantitative analysis of spectra
The spectrum of an object can reveal the object’s:
Composition
Temperature
Velocity
Four Ways in Which
Light can Interact with Matter
1.
emission – matter releases energy as light
2.
absorption – matter takes energy from light
3.
transmission – matter allows light to pass through it
4.
reflection – matter reflects light
The type of interaction is determined
by characteristics of the “matter” and
the wavelength of light.
Different
wavelengths
of light
interact
differently
with the
atmosphere
Three ways in which spectra manifest
themselves:
 Continuous spectra
 Absorption spectra
 Emission line spectra
 Continuous spectra
are usually related to the
temperature of an object that is
emitting radiation.
 Absorption & emission
line spectra are related to the
composition of the material
absorbing or emitting radiation.
Thermal Emission
A hot, dense glowing object (solid or gas)
emits a continuous spectrum.
1. Hotter objects emit
more total radiation
per unit surface
area.
2. Hotter objects have
peak emissions at
shorter wavelengths
(they will appear
“bluer”)
Energy emitted per square meter
Rules for Thermal Emission by Opaque Objects
5000 K
4000 K
3000 K
Wavelength
The sun emits its
peak radiation in
the yellow portion
of the visible
spectrum.
The human eye has its peak
sensitivity at the same wavelength.
Coincidence?
infrared
At “room
temperature”, or
“body-temperature”,
objects emit their
peak radiation in the
infrared.
The surface of the
Earth emits
radiation in the
infrared.
visible
Extremely hot objects will emit
most of their radiation in the
ultraviolet, x-ray or even the
gamma ray portion of the spectrum
“Matter” and Light
nucleus
Atom
electron
(proton,neutrons)
p+
n
●
●
●
e-
10,000,000 atoms can fit across a period in your textbook.
The nucleus is nearly 100,000 times smaller than the entire atom (if
atom filled the classroom auditorium, the nucleus would be barely
visible at its center).
Although it is the smallest part of the atom, most of the atom’s mass
is contained in the nucleus.
Electrons do not “orbit” the nucleus; they are “smeared
out” in a cloud which give the atom its size.
Incorrect
view
better
view
The number of protons in the nucleus, i.e., the
“atomic number”, determines the element
Atomic Number
1
2
3
4
5
6
7
8
Element
Hydrogen (H)
Helium (He)
Lithium (Li)
Beryllium (Be)
Boron (B)
Carbon (C)
Nitrogen (N)
Oxygen (O)
Hydrogen
ep+
atomic number = 1
atomic mass number = number protons + neutrons = 1
Helium
ep+p+
n n
eatomic number = 2
atomic mass number = number protons + neutrons = 4
Relative abundances of
elements in the universe
Every element has multiple isotopes
(same number
of protons, different numbers of neutrons) some of which
may not be stable (“radioactive”)
Carbon-14 half-life
= 5,730 yrs
Hydrogen
Deuterium
isotope
of hydrogen
atomic number = 1
p+
n
e-
atomic mass number = number protons + neutrons = 2
Unstable isotopes and radioactivity
Unstable (“radioactive”) isotopes “decay”,
producing a new type of atom, i.e., an
atom of a different element OR a different
isotope of the original element.
One half of the atoms of an
unstable isotope decay in one
“half-life” of that isotope.
Three isotopes of Carbon, two stable, one unstable.
5730 yrs
14C
 14N + electron + antineutrino + energy
Mass (14C) > Mass (14N + electron + antineutrino)
 difference in mass is converted into energy: E = mc2
What if an electron is missing?
ion
ep+p+
n n
atomic number = 2
+1
He
atomic mass number = number protons + neutrons = 4
What if two or more atoms combine to form a
particle?
molecule
H2O (water)
p+
Sharing of electrons
(chemistry) is
involved in the
construction of
molecules
8p+
8n
p+
Absorption & Emission
Line spectra
Electron Energy Levels
● Electrons cannot have just any
energy while orbiting the nucleus.
● Only certain energy values are
allowed (like the floors of an
aprtment building).
● Electrons may only gain or lose
certain specific amounts of
energy (equal to differences in
energy levels).
Electron Orbits / Absorption & Emission
●
Electrons can gain or lose energy while they orbit the nucleus.
●
When electrons have the lowest energy possible, we say the
atom is in the ground state.
●
When electrons have more energy than this, we say the atom
is in an excited state.
●
When electrons gain enough energy to escape the nucleus,
we say the atom is ionized.
• Each element
has its own
distinctive set
of energy levels
for its
electrons.
• This diagram
depicts the
energy levels of
Hydrogen.
1 eV = 1.60 x 10-19 joules
Emission/Absorption Spectra
Hydrogen
• Each electron is only allowed
to have certain energies in an
atom.
• Electrons can absorb light and
gain energy or emit light when
they lose energy.
• Only photons whose energies (colors) match the “jump” in
electron energy levels can be emitted or absorbed.
Kirchhoff’s Laws #2
2. A hot, low density gas emits light of
only certain wavelengths – an emission
line spectrum.
Kirchhoff’s Law #3
3. When light having a continuous spectrum
passes through a cool gas, dark lines appear
in the continuous spectrum – an absorption
line spectrum.
Absorption Spectra
• If light shines through a gas, each element will
absorb those photons whose energy match their
electron energy levels.
●
The resulting absorption line spectrum has all colors
minus those that were absorbed.
• We can determine which elements are present in an object
by identifying emission & absorption lines.
Molecules have rotational & vibrational
energy levels
(less energetic than electron energy levels,
energies correspond with infrared,
microwave, and radio radiations)
The Doppler Shift:
 = v

c
A shift in wavelength due to a
wave emitter moving towards
(shorter wavelength) or away
(longer wavelength) from an
observer.
The Doppler Effect
1. Light emitted from an object moving towards
you will have its wavelength shortened.
BLUESHIFT
2. Light emitted from an object moving away from
you will have its wavelength lengthened.
REDSHIFT
3. Light emitted from an object moving
perpendicular to your line-of-sight will not
change its wavelength.
Measuring Radial Velocity
●
●
We can measure the Doppler
shift of emission or absorption
lines in the spectrum of an
astronomical object.
We can then calculate the
velocity of the object in the
direction either towards or
away from Earth. (radial
velocity)
 = v

c
Measuring Rotational Velocity